The objective of this research is to use the Xenon 133 disappearance technique to measure human skin blood flow. It is the only method available for measuring skin blood flow that is potentially applicable to any skin region. It has compared favorably with venous occlusion plethysmography on normal human digital skin. Although highly quantitative methods of comparison do not exist for other skin regions (e.g., back or buttocks), skin temperature and photoelectric plethysmography may be used simultaneously to provide semi-quantitative comparison to the Xe estimate of flow rate. To measure skin blood flow by the Xe133 disappearance technique, 30 micron Ci of Xe dissolved in 0.02- 0.04 ml. physiological saline is injected into the skin with a 27 gauge needle. The gamma radiation of Xe133 is detected externally with a scintillation detector and the rate of skin blood flow is computed from the initial rate of Xe133 disappearance. Three experimental interventions will be employed to alter skin blood flow. They are: 1) irradiation of the skin with (the sunburn spectrum of) ultraviolet light, 2) topical application of steroid hormone preparations which cause the skin to blanch, and 3) local application of graded external pressure. The significance of the study will be: 1) assessment of the general usefulness of the Xenon technique, and 2) acquisition of specific data unobtainable by other methods concerning the mechanism of action of topical steroids and ultraviolet light, and determination of criteria for the design of prosthetic and orthotic devices within the range of pressure tolerance of the skin. The Xe133 technique may be used to study the peripheral circulation when edema and acrosclerosis invalidate plethysmography, and it may be useful for following the viability of skin grafts. It may be used to evaluate other drug, blanch and erythema reactions of the skin.